The adsorption of uracil on the Si(001) surface has been investigated by density-functional theory calculations using a plane-wave basis in conjunction with ultrasoft pseudopotentials. A large number of possible interface structures are studied. Electrostatic effects and the keto-enol tautomerism play an important role for the surface reaction. There exists a pronounced tendency for molecular fragmentation, leading to the dissociation of hydrogen from the molecules and possibly to oxygen insertion into Si dimers.